A sink or bath conventionally has an overflow which determines the maximum level to which water can rise in the sink bowl. Usually, the overflow is in the form of an opening in the wall of the bowl, the opening being connected to the waste pipe either by means of a channel formed in the body of the sink itself or by means of a flexible hose interconnecting an overflow terminal and a terminal on the waste system.
Such overflow systems are simple and well understood, but they do have some disadvantages. An overflow channel within the sink body adds complexity to the structure and to the manufacturing process. A separate flexible hose arrangement uses a number of components and can be awkward to install and maintain. In addition, the waste opening in the wall of the sink bowl presents potential hygiene problems and can be difficult to clean.
It has hitherto been proposed to provide an overflow which is concealed, in the sense that there is no overflow opening in the wall of the bowl, and no overflow arrangement is visible to the user. Prior proposals, however, have relied on the use of valves in various forms, with consequent complexities in manufacture and potential reliability problems.
Additionally, the requirement for a water seal and to the height of the construction in the space, particularly when you're talking about baths is particularly limited. Furthermore there is also issues associated with drainage of these devices, and particularly the speed of drainage, due to the complicated construction.